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United States Patent |
5,709,282
|
Akira
,   et al.
|
January 20, 1998
|
Traveling control system for hydraulically driven vehicle
Abstract
A traveling control system for a hydraulically driven vehicle enables the
vehicle to travel at a higher speed and also enables a vehicle body to be
braked appropriately when the vehicle descends a slope with a travel
control valve kept in its neutral position. The traveling control system
includes pressure switches for detecting an operative state of a travel
control valve, a detection line for detecting a shift position of a
transmission, and a controller, a solenoid valve, load lines, a shuttle
valve and a control line for cooperatively increasing the capacity of a
travel motor upon the detection of that the travel control valve is in its
neutral position and also the transmission is shifted to a high-speed
gear. A timer function of the controller allows the capacity of the travel
motor to be increased after the elapse of a predetermined time after the
detection of that the travel control valve is in the neutral position. The
load lines and the shuttle valve jointly serve as a hydraulic source for
switching the motor capacity.
Inventors:
|
Akira; Tatsumi (Ibaraki-ken, JP);
Duri; Gianni (Bologna, IT);
Prealta; Dario (Torino, IT)
|
Assignee:
|
Fiat-Hatachi Excavators S.P.A. (San Mauro Torinese, IT)
|
Appl. No.:
|
625588 |
Filed:
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March 28, 1996 |
Foreign Application Priority Data
Current U.S. Class: |
180/307; 60/445 |
Intern'l Class: |
B60K 026/00 |
Field of Search: |
180/305,306,307,308,242
60/444,451,447,445
|
References Cited
U.S. Patent Documents
4240515 | Dec., 1980 | Kirkwood | 180/308.
|
4241577 | Dec., 1980 | Baldauf | 180/307.
|
4350220 | Sep., 1982 | Carman | 180/308.
|
4530416 | Jul., 1985 | Kasai | 180/307.
|
4554992 | Nov., 1985 | Kassai | 180/307.
|
Foreign Patent Documents |
63-54521 | Apr., 1988 | JP | .
|
Primary Examiner: Hurley; Kevin
Attorney, Agent or Firm: Miller; Larry W., Stader; John W., Seemar; Frank A.
Claims
Having thus described the invention, what is claimed is:
1. In a hydraulically driven vehicle having a hydraulic pump driven by a
prime mover; a variable displacement hydraulic travel motor driven by a
hydraulic fluid supplied from the hydraulic pump; a travel control valve
for controlling a flow rate of the hydraulic fluid supplied from said
hydraulic pump to said travel motor; operation means for operating said
travel control valve; and first motor capacity control means for
increasing the capacity of said travel motor when the load pressure of
said travel motor becomes high during operation of said travel control
valve, an improved traveling control system comprising:
first detecting means for detecting an operative state of said travel
control valve, and
second motor capacity control means for increasing the capacity of said
travel motor when said first detecting means detects that said travel
control valve is in its neutral position.
2. The hydraulically driven vehicle according to claim 1, further
comprising a transmission provided in an output section of said travel
motor and capable of shifting between a high-speed gear and a low-speed
gear, and second detecting means for detecting a shift position of said
transmission, said second motor capacity control means increasing the
capacity of said travel motor when said first detecting means detects that
said travel control valve is in its neutral position and also said second
detecting means detects that said transmission is shifted to the
high-speed gear.
3. The hydraulically driven vehicle according to claim 1 wherein said
second motor capacity control means includes delay means for allowing the
capacity of said travel motor to be increased after the elapse of a
predetermined time after the detection of that said travel control valve
is in its neutral position.
4. The hydraulically driven vehicle according to claim 1, wherein said
second motor capacity control means comprises a hydraulic source, valve
means for switching communication between said hydraulic source and a
hydraulic actuator for driving a mechanism of varying the capacity of said
travel motor, and a controller for operating said valve means to
communicate said hydraulic source with said hydraulic actuator when said
first detecting means detects that said travel control valve is in its
neutral position.
5. The hydraulically driven vehicle according to claim 1, wherein said
first detecting means is means for detecting operation signals for both
forward and backward traveling from said operation means.
6. The hydraulically driven vehicle according to claim 1, wherein said
first detecting means is means for detecting an operation signal for
forward traveling only from said operation means, throttles for delaying
return of said travel control valve from forward and backward traveling
positions to the neutral position are disposed in respective pilot lines
for transmitting the operation signals for forward and backward traveling
to said travel control valve therethrough, and an aperture of said
throttle for delaying return of said travel control valve from the forward
traveling position is set to be smaller than an aperture of said throttle
for delaying return of said travel control valve from the backward
traveling position.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a traveling control system for
hydraulically driven vehicles such as hydraulic excavators, and more
particularly to a traveling control system for hydraulically driven
vehicles which comprises a variable displacement hydraulic travel motor as
a drive source for traveling, and which controls a traveling speed by
automatically changing the capacity of the travel motor depending on the
traveling load.
As a traveling control system for hydraulically driven vehicles such as
hydraulic excavators, there is hitherto known one comprising a hydraulic
pump driven by a prime mover, a traveling hydraulic circuit including a
variable displacement hydraulic travel motor driven by a hydraulic fluid
supplied from the hydraulic pump and a travel control valve for
controlling a flow rate of the hydraulic fluid supplied from the hydraulic
pump to the travel motor, and motor capacity control means for increasing
the capacity (tilting) of the travel motor when the load pressure of the
travel motor becomes high during operation of the travel control valve, as
described in JP, U, 63-54521.
In this traveling control system, when the vehicle is running with a light
traveling load as experienced in traveling over level ground, for example,
and hence the load pressure of the travel motor is small, the capacity
(tilting) of the travel motor is controlled to be small so that the
vehicle can travel at a high speed (with a small torque). On the other
hand, when the vehicle is running with a heavy traveling load as
experienced in traveling over an upward slope or under acceleration, for
example, and hence the load pressure of the travel motor is high, the
capacity (tilting) of the travel motor is controlled to be high so that
the vehicle can travel (at a low speed) with a large torque while
producing sufficient tractive force to ascend a slope.
Further, a brake valve is usually provided between the travel motor and the
travel control valve in the traveling hydraulic circuit. When the vehicle
is traveling under deceleration or descending a slope with the travel
control valve kept in its neutral position, the circuit section between
the brake valve and the travel motor forms a closed circuit, enabling the
vehicle to speed down through a throttle of the brake valve and the set
pressure of a relief valve.
To prevent a hunting in capacity control, pressure detection for the
capacity control of the travel motor is usually made in the circuit
section between the brake valve and the travel control valve. Therefore,
when the vehicle travels with no pedal trod down and the travel control
valve kept in its neutral position, the circuit section between the travel
control valve and the brake valve is held under the reservoir pressure
through the travel control valve and, as a result, the travel motor is
controlled to the minimum capacity.
Additionally, a transmission capable of shifting between a high-speed gear
and a low-speed gear is provided in an output section of the travel motor
so that the traveling speed is switched over in two stages given by the
high-speed gear and the low-speed gear upon operation of a changeover
switch.
SUMMARY OF THE INVENTION
Recently, there has been a need for increasing speeds of hydraulically
traveling vehicles. For realization of higher speeds, it is desired to
diminish the pressure loss in the hydraulic circuit by increasing the
hydraulic pressure and reducing the flow rate, taking into account input
torque limiting control of the hydraulic pump. In this case, corresponding
to a reduction in the flow rate, the minimum capacity of the travel motor
must be set to be small, or the speed reducing ratio of the transmission
must be set to be small for ensuring the traveling speed. Thus, the
driving force is offset by the increased pressure. Increasing speeds of
hydraulically traveling vehicles can also be realized by setting the speed
reducing ratio of the transmission to be small.
Where the minimum capacity of the travel motor is set to be small, or the
speed reducing ratio of the transmission is set to be small as mentioned
above, no problems occur in ordinary traveling such as traveling over
level ground and an upward slope, but the following problem is caused when
the vehicle descends a slope with no pedal trod down and the travel
control valve kept in its neutral position, i.e., without applying any
driving force to the travel motor.
More specifically, when the vehicle descends a slope with no pedal trod
down and the travel control valve kept in its neutral position, the
circuit section between the brake valve and the travel motor forms a
closed circuit so that the vehicle is sped down through the throttle of
the brake valve and the set pressure of the relief valve, as explained
above. At this time, however, because the circuit section between the
travel control valve and the brake valve is held under the reservoir
pressure through the travel control valve as mentioned above, the travel
motor is controlled to the minimum capacity.
Also, at this time, because the minimum capacity of the travel motor is set
to be small, or the speed reducing ratio of the transmission is set to be
small for the above-mentioned reason, sufficient braking force cannot be
produced and the vehicle body cannot be stopped or braked satisfactorily.
Further, the temperature of the hydraulic fluid in the circuit may rise so
high as to bring about a risk of damaging hydraulic equipment.
An object of the present invention is to provide a traveling control system
for a hydraulically driven vehicle with which the vehicle can travel at a
higher speed and a vehicle body can be braked appropriately when the
vehicle descends a slope with a travel control valve kept in its neutral
position.
To achieve the above object, the present invention is constituted as
follows. Specifically, in a traveling control system for a hydraulically
driven vehicle comprising a hydraulic pump driven by a prime mover, a
variable displacement hydraulic travel motor driven by a hydraulic fluid
supplied from the hydraulic pump, a travel control valve for controlling a
flow rate of the hydraulic fluid supplied from the hydraulic pump to the
travel motor, operation means for operating the travel control valve, and
first motor capacity control means for increasing the capacity of the
travel motor when the load pressure of the travel motor becomes high
during operation of the travel control valve, the traveling control system
further comprises first detecting means for detecting an operative state
of the travel control valve, and second motor capacity control means for
increasing the capacity of the travel motor when the first detecting means
detects that the travel control valve is in its neutral position.
Preferably, the above traveling control system for a hydraulically driven
vehicle further comprises a transmission provided in an output section of
the travel motor and capable of shifting between a high-speed gear and a
low-speed gear, and second detecting means for detecting a shift position
of the transmission, the second motor capacity control means increasing
the capacity of the travel motor when the first detecting means detects
that the travel control valve is in its neutral position and also the
second detecting means detects that the transmission is shifted to the
high-speed gear.
Also preferably, the second motor capacity control means includes delay
means for allowing the capacity of the travel motor to be increased after
the elapse of a predetermined time after the detection of that the travel
control valve is in its neutral position.
Further preferably, the second motor capacity control means comprises a
hydraulic source, valve means for switching communication between the
hydraulic source and a hydraulic actuator for driving a mechanism of
varying the capacity of the travel motor, and a controller for operating
the valve means to communicate the hydraulic source with the hydraulic
actuator when the first detecting means detects that the travel control
valve is in its neutral position.
Still further preferably, the first detecting means is means for detecting
operation signals for both forward and backward traveling from the
operation means.
The first detecting means may be means for detecting an operation signal
for forward traveling only from the operation means. In this case,
preferably, throttles for delaying return of the travel control valve from
forward and backward traveling positions to the neutral position are
disposed in respective pilot lines for transmitting the operation signals
for forward and backward traveling to the travel control valve
therethrough, and an aperture of the throttle for delaying return of the
travel control valve from the forward traveling position is set to be
smaller than an aperture of the throttle for delaying return of the travel
control valve from the backward traveling position.
In the present invention arranged as set forth above, since the second
motor capacity control means makes control to increase the capacity of the
travel motor when the first detecting means detects that the travel
control valve is in the neutral position, the travel motor is switched to
the large capacity when the vehicle descends a slope with no pedal trod
down and the travel control valve kept in the neutral position.
Therefore, in the case that the control system is designed to enable the
vehicle to travel at a higher speed than conventional when running with a
light traveling load as experienced in traveling over level ground, for
example, by setting the minimum tilting of the travel motor to be smaller
than conventional, or setting the speed reducing ratio of the transmission
to be smaller than conventional, hydraulic braking force can be increased
to brake the vehicle body appropriately in downslope traveling, and an
excessive temperature rise of the hydraulic fluid in the circuit can be
prevented. It is hence possible to prevent damages of hydraulic equipment
otherwise caused by an excessive temperature rise of the hydraulic fluid
in the circuit.
With the feature that the second motor capacity control means makes control
to increase the capacity of the travel motor when the first detecting
means detects that the travel control valve is in the neutral position and
also the transmission is shifted to the high-speed gear, in the case of
the transmission shifted to the high-speed gear, the travel motor is
switched to the large capacity when the vehicle descends a slope with no
pedal trod down and the travel control valve kept in the neutral position,
whereby the hydraulic braking force can be increased to brake the vehicle
body appropriately as mentioned above. On the other hand, in the case of
the transmission shifted to the low-speed gear, the travel motor is not
switched to the large capacity. However, predetermined braking force is
produced through the low-speed gear of the transmission, and excessive
speed-down otherwise caused at the low-speed gear because of the motor
capacity being set to a small value is avoided so as to prevent the
worsening of a speed-down feeling.
With the feature that the second motor capacity control means includes
delay means for allowing the capacity of the travel motor to be increased
after the elapse of a predetermined time after the detection of that the
travel control valve is in its neutral position, a speed-down feeling at
the high-speed gear just after the travel control valve is returned to the
neutral position is improved, and the cavitation otherwise caused upon
switching of the travel motor to the large tilting is prevented.
With the feature that the second motor capacity control means comprises a
hydraulic source, valve means for switching communication between the
hydraulic source and a hydraulic actuator for driving a mechanism of
varying the capacity of the travel motor, and a controller for operating
the valve means to communicate the hydraulic source with the hydraulic
actuator when the first detecting means detects that the travel control
valve is in its neutral position, the second motor capacity control means
can be constructed in electro-hydraulic fashion.
With the feature that the first detecting means is means for detecting
operation signals for both forward and backward traveling from the
operation means, the travel motor is switched to the large capacity
whenever the travel control valve is returned to the neutral position from
any of the forward and backward traveling positions. Therefore, the same
speed-down feeling can be provided in both forward and backward traveling.
When the first detecting means comprises means for detecting the operation
signal for forward traveling only from the operation means, a difference
in speed-down feeling between forward and backward traveling perceived
when the travel control valve is returned to the neutral position can be
reduced with the feature that an aperture of the throttle for delaying
return of the travel control valve from the forward traveling position is
set to be smaller than an aperture of the throttle for delaying return of
the travel control valve from the backward traveling position. Also, a
reduction in the production cost of the system can be achieved because a
sensor for detecting the operation signal for forward traveling only is
required in this case.
BRIEF DESCRIPTION OF THE DRAWINGS
A traveling control system for a hydraulically driven vehicle in accordance
with the present invention will now be described, by way of example, with
reference to the accompanying drawings, wherein:
FIG. 1 is an overall schematic diagram of a traveling control system for a
hydraulically driven vehicle according to a first embodiment of the
present invention;
FIG. 2 is a diagram showing details of a regulator for a main pump shown in
FIG. 1;
FIG. 3 is a diagram showing processing functions of a controller shown in
FIG. 3;
FIG. 4 is a graph showing changes in pressure loss and traveling speed
resulted when the traveling flow rate is reduced;
FIG. 5 is a graph showing characteristics of an input torque limiting
control function of the regulator shown in FIG. 1; and
FIG. 6 is an overall schematic diagram of a traveling control system for a
hydraulically driven vehicle according to a second embodiment of the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Embodiments of the present invention will be described hereinafter with
reference to the drawings. A first embodiment of the present invention
will be described with reference to FIGS. 1 to 5.
In FIG. 1, a traveling control system for a hydraulically driven vehicle of
this embodiment includes an engine 1, a main hydraulic source 4 consisting
of a variable displacement main pump 2 driven by the engine 1 and a
regulator 3 for moving a swash plate 2a of the main pump 2 to control a
tilting angle (displacement volume). A pilot hydraulic source 6 consists
of two pilot pumps 5a, 5b and a relief valve (not shown) for setting an
upper limit of pilot pressure. A traveling drive circuit 8 includes a
variable displacement hydraulic travel motor 7 driven by a hydraulic fluid
supplied from the main pump 2, and a travel control valve 9 incorporating
a variable throttle to control a flow rate of the hydraulic fluid supplied
from the main pump 2 to the travel motor 7.
A pressure compensating valve 10 includes a maximum load pressure detecting
mechanism and is disposed downstream of the variable throttle in the
travel control valve 9 for controlling a differential pressure across the
variable throttle to be substantially constant. A traveling pilot valve
device 11 is operated upon an operator treading a pedal (not shown) for
producing a pilot pressure corresponding to the amount by which the pedal
is operated, by the use of a hydraulic fluid from the pilot pump 5a. A
pilot operation circuit 12 transmits the produced pilot pressure to an
operative sector 9a, 9b of the travel control valve 9.
A transmission 13 disposed in an output section of the travel motor 7 is
operable of shifting between a high-speed gear and a low-speed gear with
operation of a not-shown hydraulic cylinder. A transmission shifting
device 14 is provided for selectively introducing the hydraulic fluid from
the pilot pump 5a to the hydraulic cylinder in the transmission 13 for
shifting the transmission 13 to one of the high-speed gear and the
low-speed gear. An LS line 15 for transmitting the maximum load pressure
detected by the pressure compensating valve 10 loads sensing pressure
(hereinafter abbreviated to LS pressure) regulator 3. A pump cutoff relief
valve 16 limits a maximum delivery pressure of the main pump 2, and an LS
main relief valve 17 limits an upper limit of the LS pressure in the LS
line 15.
The regulator 3 comprises, as shown in FIG. 2, a control hydraulic actuator
3a for driving the swash plate 2a of the main pump 2, a first servo valve
3b for LS control for controlling a flow rate of the hydraulic fluid
supplied to the hydraulic actuator 3a in response to the LS pressure
introduced through the LS line 15 and then controlling the tilting angle
of the swash plate 2a (the displacement volume of the main pump 2), and a
second servo valve 3c for input torque limiting control for controlling a
flow rate of the hydraulic fluid supplied to the hydraulic actuator 3a in
response to the delivery pressure of the main pump 2 itself and then
controlling the tilting angle of the swash plate 2a (the displacement
volume of the main pump 2).
The traveling drive circuit 8 comprises a pair of main lines 20a, 20b for
connecting the travel motor 7 to the travel control valve 9, a brake valve
21 and overload relief valves 22a, 22b disposed between the main lines
20a, 20b, and first motor capacity control means 23 for, during operation
of the travel control valve 9, holding the travel motor 7 at a small
tilting (small capacity) when the load pressure of the travel motor 7 is
small, and switching the travel motor 7 to a large tilting (large
capacity) when the load pressure of the travel motor 7 becomes large.
Here, for the purpose of increasing the speed of the hydraulically
traveling vehicle, the minimum tilting (minimum capacity) of the travel
motor 7 is set to be smaller than that of a conventional travel motor, and
the first motor capacity control means switches the travel motor 7 to that
minimum tilting when the load pressure of the travel motor 7 is small. For
example, the minimum tilting (minimum capacity) is 100 cc/rev for the
conventional travel motor, whereas it is 50 cc/rev for the travel motor 7
of this embodiment.
The travel control valve 9 is of normally open type constructed such that
the sections of the main lines 20a, 20b between the travel control valve 9
and the brake valve 21 are communicated with a reservoir 24 when the
travel control valve 9 is in its neutral position, and the hydraulic fluid
in the reservoir 24 can be resupplied when the sections of the main lines
20a, 20b between the brake valve 21 and the travel motor 7 are subjected
to negative pressure.
The brake valve 21 is a so-called counter balance valve and has a neutral
position and also a left and right open positions. Throttles 21a, 2lb are
provided in parallel to the brake valve 21. Under a running condition
where the travel motor 7 undergoes a negative load as experienced in
traveling over a downward slope, for example, the brake valve 21 is
returned to the neutral position so that a braking pressure is produced in
the main line 20a or 20b on the delivery side of the travel motor 7
through the throttle 21a, 21b and the overload relief valve 22a, 22b.
The first motor capacity control means 23 comprises a control hydraulic
cylinder 23a for moving the swash plate 7a of the travel motor 7 and
switching the capacity of the travel motor 7, a shuttle valve 23b for
selectively taking out higher one of load pressures in the main lines 20a,
20b, a control line 23c for introducing the load pressure taken out by the
shuttle valve 23b, as a working pressure, to the rod side of the hydraulic
cylinder 23a, and a switching valve 23d for communicating the bottom side
of the hydraulic cylinder 23a to the reservoir 24 when the load pressure
introduced to the control line 23c is low, and communicating the bottom
side of the hydraulic cylinder 23a to the control line 23c when the load
pressure introduced to the control line 23c becomes high.
The hydraulic cylinder 23a contracts due to the hydraulic fluid introduced
to the rod side when the bottom side is under the reservoir pressure,
thereby switching the travel motor 7 to the minimum tilting (minimum
capacity; hereinafter referred to as small tilting or small capacity), and
it extends due to an area difference between the bottom side and the rod
side when the load pressure in the control line 23c is introduced to the
bottom side, thereby switching the travel motor 7 to the maximum tilting
(maximum capacity; hereinafter referred to as large tilting or large
capacity).
With this arrangement, when the vehicle is running with a light traveling
load as experienced in traveling over level ground, for example, the load
pressure of the travel motor 7 is small and, therefore, the travel motor 7
is switched to the small tilting, enabling the vehicle to travel at a high
speed (with a small torque). When the vehicle is running with a heavy
traveling load as experienced in traveling over an upward slope or under
acceleration, for example, the load pressure of the travel motor 7 is high
and, therefore, the travel motor is switched to the large tilting,
enabling the vehicle to travel (at a low speed) with a large torque.
The pilot operation circuit 12 comprises a pilot line 12a for transmitting
the pilot pressure produced by the traveling pilot valve device 11 for
forward traveling to the operative sector 9a of the travel control valve
9, and a pilot line 12b for transmitting the pilot pressure produced by
the traveling pilot valve device 11 for backward traveling to the
operative sector 9b of the travel control valve 9.
A slow return valve 12e consisted of a throttle 12c and a check valve 12d
for slowing return of the travel control valve 9 from the forward
traveling position to the neutral position to moderate a shock caused upon
the vehicle stopping or under deceleration is disposed in the pilot line
12a, while a throttle 12f for slowing shift of the travel control valve 9
from the neutral position to the backward traveling position and return
thereof from the backward traveling position to the neutral position to
moderate shocks caused upon the vehicle starting backward or under
acceleration and upon the vehicle stopping or under deceleration is
disposed in the pilot line 12f. Both the throttles 12c and 12f have the
same aperture, e.g., 1.4 mm.
The transmission shifting device 14 comprises a power supply 14a, a
low-speed gear select switch 14b, and a solenoid valve 14c maintained in
the illustrated position when the low-speed gear select switch 14b is
opened, and excited when the switch 14b is operated to be closed, for
shifting from the illustrated position to the opposite position. When the
solenoid valve 14c is in the illustrated position, the not-shown hydraulic
cylinder for gear shifting in the transmission 13 is communicated with the
reservoir 24, whereupon the transmission 13 is shifted to the highspeed
gear. When the low-speed gear select switch 14b is operated and the
solenoid valve 14 is shifted from the illustrated position to the opposite
position, the hydraulic fluid is supplied from the pilot pump 5b to the
hydraulic cylinder for gear shifting in the transmission 13, whereupon the
transmission 13 is shifted to the low-speed gear.
As other features in the arrangement, the traveling control system of this
embodiment further comprises a pressure switch 30a connected to the pilot
line 12a in the pilot operation circuit 12 and turning on when the pilot
pressure for forward traveling, that is produced as pressure on the
secondary side of the traveling pilot valve device 11, exceeds a value
capable of shifting the travel control valve 9 from the neutral position
to the operative position, a pressure switch 30b connected to the pilot
line 12b in the pilot operation circuit 12 and turning on when the pilot
pressure for backward traveling, that is produced as a pressure on the
secondary side of the traveling pilot valve device 11, exceeds a value
capable of shifting the travel control valve 9 from the neutral position
to the operative position, a detection line 31 connected to a junction
between the low-speed gear select switch 14b and the solenoid valve 14c in
the transmission shifting device 14 for taking out a signal from the
low-speed gear select switch 14b, a controller 32 for receiving signals
from the pressure switches 30a, 30b and the signal sent through the
detection line 31 and executing a predetermined sequence of processing,
load lines 34a, 34b and a shuttle valve 35 connected to the main lines
20a, 20b at junctions between the travel motor 7 and the brake valve 21
for selectively taking out higher one of load pressures of the travel
motor 7, a control line 36 for transmitting the load pressure taken out by
the shuttle valve 35, as working pressure, to the control line 23c, a
solenoid valve 33 disposed in the control line 36 and driven by a signal
from the controller 32, and a check valve 37 for, when the load pressure
transmitted from the control line 36 to the control line 23c is higher
than the pressure taken out by the shuttle valve 23b, preventing that
higher load pressure from being transmitted to the shuttle valve 23b.
When the drive signal from the controller 32 is on, the solenoid valve 33
is shifted to a closed position on the left side in the drawing where the
control line 36 is disconnected, thereby cutting off the communication
between the shuttle valve 35 and the control line 23c. When the drive
signal from the controller 32 is turned off, the solenoid valve 33 is
shifted to an operative position on the right side in the drawing,
whereupon the shuttle valve 35 is communicated with the control line 23c
through the control line 36 and a throttle 33a incorporated in the
solenoid valve 33. The throttle 33a has a diameter of about 0.6 mm, for
example, and serves as a hydraulic timer.
Processing functions of the controller 32 are shown in a functional block
diagram of FIG. 3. The controller 32 to the closed position on the left
side in the drawing.
Where the low-speed gear select switch 14b is made open and the
transmission 13 is shifted to the high-speed gear, the signal from the
lowspeed gear select switch 14b is off. At this time, if one of the travel
pressure switches 30a, 30b is turned on, the OR function 32c outputs an
on-signal similarly to the above case so that the solenoid valve 33 is
shifted to the closed position on the left side in the drawing. On the
other hand, if the travel pressure switches 30a, 30b are both turned off
with the signal from the low-speed gear select switch 14b kept off, the
output of the OR function 32c is turned off after the elapse of a
predetermined time set by the timer 32b, e.g., 1.5 second, so that the
solenoid valve 33 is shifted to the operative position on the right side
in the drawing where the function of the throttle 33a is effected.
In the above arrangement, the pressure switches 30a, 30b constitute first
detecting means for detecting the operative state of the travel control
valve 9. The controller 32, the solenoid valve 33, the load lines 34a,
34b, the shuttle valve 35 and the control line 36 constitute second motor
capacity control means for increasing the capacity of the travel motor 7
when the first detecting means 30a, 30b detects that the travel control
valve 9 is in the neutral position.
Also, the detection line 31 constitutes second detecting means for
detecting the shift position of the transmission 13. The above second
motor capacity control means 32, 33, 34a, 34b, 35, 36 makes control to
increase the capacity of the travel motor 7 when the first detecting means
30a, 30b detects that the travel control valve 9 is in the neutral
position and the second detecting means 31 detects that the transmission
13 is on the high-speed gear side.
Further, the timer function 32b of the controller 32 constitutes delay
means for increasing the capacity of the travel motor 7 after the elapse
of a predetermined time from the detection of that the travel control
valve 9 is in the neutral position. Additionally, the load lines 34a, 34b
and the shuttle valve 35 constitute a hydraulic source for the second
motor capacity control means.
The operation of this embodiment thus arranged will now be described below.
First, in ordinary traveling such as traveling over level ground and an
upward slope, the travel control valve is shifted from the neutral
position with the pilot pressure in the pilot line 12a or 12b of the pilot
operation circuit 12, one of the pressure switches 30a, 30b is turned on,
and an on-signal is output from the OR function 32c of the controller 32
for shifting the solenoid valve 33 to the closed position on the left side
in FIG. 1. Therefore, the communication between the control line 23c of
the first motor capacity control means 23 and the shuttle valve 35 of the
second motor capacity control means is cut off, whereby the load pressure
taken out by the shuttle valve 23b is introduced to the control line 23c.
When the vehicle is running with a light traveling load as experienced in
traveling over level ground, for example, of ordinary traveling, the load
pressure of the travel motor 7 is small. Therefore, the switching valve
23d of the first motor capacity control means 23 communicates the bottom
side of the hydraulic cylinder 23a with the reservoir 24, and the
hydraulic cylinder 23a switches the travel motor 7 to the small tilting
(small capacity), enabling the vehicle to travel at a high speed (with a
small torque).
In this embodiment, as mentioned before, the minimum tilting (minimum
capacity) of the travel motor 7 is set to be smaller than that of the
conventional travel motor so that when the vehicle is running with a light
traveling load as experienced in traveling over level ground, for example,
it can travel at a higher speed than a conventional hydraulically
traveling vehicle. This feature will be described below in detail.
As mentioned before, in this embodiment, the minimum tilting (minimum
capacity) of the travel motor 7 is reduced to 50 cc/rev as compared with
100 cc/rev in the conventional travel motor. Here, the number of
revolutions of the travel motor 7 (i.e., the traveling speed) is obtained
by dividing the motor capacity by the traveling flow rate. Accordingly,
the traveling speed must be the same if the flow rate of the hydraulic
fluid supplied from the main pump 2 to the travel motor 7, i.e., the
traveling flow rate, is reduced in proportion to a lowering of the motor
capacity.
FIG. 4 shows the relationship between the pressure loss in the circuit and
the traveling speed which are changed when the minimum tilting (minimum
capacity) of the travel motor 7 is continuously reduced from 100 cc/rev to
50 cc/rev and, correspondingly, the flow rate of the hydraulic fluid
supplied from the main pump 2 to the travel motor 7 (i.e., the traveling
flow rate) is continuously reduced from 200 liter/min to 100 liter/min.
Since the number of revolutions of the travel motor 7 corresponds to a
value resulted by dividing the motor capacity by the traveling flow rate
as stated above, the number of revolutions of the travel motor 7 (i.e.,
the traveling speed) must be the same if the minimum tilting (minimum
capacity) of the travel motor 7 and the traveling flow rate are reduced in
proportion. In practice, however, as a result of that the pressure loss in
the circuit is reduced as the traveling flow rate lowers, the number of
revolutions of the travel motor 7 (i.e., the traveling speed) is increased
as the traveling flow rate reduces, as will be seen from FIG. 4.
On the other hand, a lowering of the driving force corresponding to the
reduction in the traveling flow rate must be offset by increasing the
hydraulic pressure. As explained above in connection with FIG. 2, the
regulator 3 for the main pump 2 in this embodiment has the input torque
limiting control function effected by the second servo valve 3c. With such
a function, the maximum allowable delivery flow rate of the main pump 2 is
controlled, as shown in FIG. 5, such that it is reduced as the delivery
pressure of the main pump 2 increases. For example, the maximum allowable
delivery flow rate is q1 when the delivery pressure of the main pump 2 is
P1, but it is reduced to q2 if the delivery pressure of the main pump 2
increases to P2. Accordingly, it is possible to reduce the traveling flow
rate and also increase the hydraulic pressure by utilizing the input
torque limiting control function of the regulator 3.
Thus, the hydraulically traveling vehicle can travel at a higher speed with
smaller pressure loss by increasing the hydraulic pressure and reducing
the traveling flow rate so as to lessen the pressure loss in the circuit,
and also by setting the minimum capacity of the travel motor 7 to a
smaller value so as to offset a reduction in the traveling flow rate and
ensure the traveling speed.
Meanwhile, when the vehicle is running with a heavy traveling load as
experienced in traveling over an upward slope or under acceleration, for
example, the load pressure of the travel motor becomes high. Therefore,
the switching valve 23d communicates the bottom side of the hydraulic
cylinder 23a with the control line 23c, and the hydraulic cylinder 23a
switches the travel motor 7 to the large tilting (large capacity),
enabling the vehicle to travel (at a low speed) with a large torque while
producing sufficient tractive force to ascend a slope.
The above capacity switching control of the travel motor 7 is carried out
likewise regardless of the shift position of the transmission 13.
Next, when the vehicle descends a slope with no pedal trod down and the
travel control valve 9 kept in the neutral position, i.e., without
applying any driving force to the travel motor 7, under the condition
where the low-speed gear select switch 14b of the transmission shifting
device 14 is closed and the transmission 13 is shifted to the low-speed
gear, the signal from the low-speed gear select switch 14b is on, an
on-signal is output from the OR function 32c of the controller 32, and the
solenoid valve 33 is shifted to the closed position on the left side in
FIG. 1.
Therefore, the communication between the control line 23c of the first
motor capacity control means 23 and the shuttle valve 35 of the second
motor capacity control means is cut off, whereby the pressure taken out by
the shuttle valve 23b is introduced to the control line 23c. At this time,
because the travel control valve 9 of normally open type is in the neutral
position, the sections of the main lines 20a, 20b between the travel
control valve 9 and the brake valve 21 are communicated with the reservoir
24. Those main line sections are thus under the reservoir pressure that is
also introduced to the control line 23c.
Therefore, the switching valve 23d of the first motor capacity control
means 23 communicates the bottom side of the hydraulic cylinder 23a with
the reservoir 24, and the hydraulic cylinder 23a switches the travel motor
7 to the small tilting (small capacity). In addition, because the brake
valve 21 is returned to the neutral position, the circuit section between
the brake valve 21 and the travel motor 7, including corresponding parts
of the main lines 20a, 20b, forms a closed circuit to produce a braking
pressure in the main line 20a or 20b on the delivery side of the travel
motor 7 through the throttle 21a, 21b and the overload relief valve 22a,
22b.
Accordingly, even if the travel motor 7 is switched to the small tilting as
mentioned above, predetermined braking force is produced since the
low-speed gear select switch 14b is closed and the transmission 13 is
shifted to the low-speed gear. As a result, the vehicle body can be
stopped or braked appropriately. Furthermore, the fact that the travel
motor 7 is switched to the small tilting is rather effective to prevent
the worsening of a speed-down feeling otherwise occurred at the low-speed
gear.
On the other hand, when the vehicle descends a slope with the travel
control valve kept in the neutral position similarly to the above case,
but under the condition where the low-speed gear select switch 14b of the
transmission shifting device 14 is made open and the transmission 13 is
shifted to the high-speed gear, the signals from the pressure switches
30a, 30b are both off and the signal from the low-speed gear select switch
14b is also off. Hence, an off-signal is output from the OR function 32c
of the controller 32 and the solenoid valve 33 is shifted to the operative
position on the right side in FIG. 1 where the throttle 33a is disposed.
Therefore, the communication between the control line 23c of the first
motor capacity control means 23 and the shuttle valve 35 of the second
motor capacity control means is established, whereby the pressure taken
out by the shuttle valve 35 is introduced to the control line 23c. At this
time, because the brake valve 21 is returned to the neutral position,
there produces a braking pressure in the main line 20a or 20b on the
delivery side of the travel motor 7 and this high braking pressure is
introduced to the control line 23c. Accordingly, the switching valve 23d
of the first motor capacity control means 23 communicates the bottom side
of the hydraulic cylinder 23a with the control line 23c, and the hydraulic
cylinder 23a switches the travel motor 7 to the large tilting (large
capacity), thereby increasing hydraulic braking force imposed on the
travel motor 7.
In this embodiment, as mentioned before, the minimum tilting (minimum
capacity) of the travel motor 7 is set to be smaller than that of the
conventional travel motor for the purpose of increasing the speed of the
hydraulically traveling vehicle. Therefore, if the travel motor 7 is
switched to the small tilting as with the above case of the transmission
13 being shifted to the low-speed gear side when the vehicle descends a
slope with the travel control valve kept in the neutral position under the
condition where the transmission 13 is shifted to the high-speed gear, as
mentioned above, sufficient braking force could not be produced and the
vehicle body could not be stopped or braked satisfactorily. This may raise
the temperature of the hydraulic fluid in the circuit so high as to bring
about a risk of damaging hydraulic equipment.
As explained above, in this embodiment, the travel motor 7 is switched to
the large tilting (large capacity), thereby increasing hydraulic braking
force imposed on the travel motor 7. Accordingly, even if the transmission
13 is shifted to the high-speed gear, predetermined braking force is
produced and the vehicle body can be stopped or braked satisfactorily.
This is effective to suppress generation of heat in the circuit between
the brake valve 21 and the travel motor 7, including corresponding parts
of the main lines 20a, 20b. In some cases, it is also possible to prompt
the operator to tread the pedal down so that the cold hydraulic fluid is
newly supplied to the circuit to prevent an excessive temperature rise of
the hydraulic fluid in the circuit.
Immediately after the running mode is shifted from ordinary traveling to
downslope traveling and the travel control valve 9 is returned to the
neutral position while the transmission 13 is kept on the high-speed gear
side, the presence of the timer function 32b in the controller 32 allows
the output of the OR function 32c to turn off after the elapse of a
predetermined time, e.g., 1.5 second, set by the timer 32b from the
turning from an on-state to an offstate of the signal from the travel
pressure switch 30a or 30b. The solenoid valve 33 is shifted to the
operative position on the right side in the drawing with such a time lag.
Further, since the throttle 33a as a hydraulic timer exists in the
operative position of the solenoid valve 33, the travel motor 7 is
prevented from switching to the large tilting abruptly when the solenoid
valve 33 is shifted to the operative position. It is therefore possible to
avoid an abrupt speed-down just at the time the travel control valve 9 is
returned to the neutral position, improve a speed-down feeling at the
high-speed gear, and prevent the cavitation otherwise caused upon
switching of the travel motor 7 to the large tilting.
With this embodiment, as described above, since the minimum tilting of the
travel motor 7 is set to be smaller than that of the conventional travel
motor, the vehicle can travel at a higher speed than conventional when
running with a light traveling load as experienced in traveling over level
ground, for example. Also, since the travel motor 7 is switched to the
large tilting when the vehicle descends a slope with no pedal trod down
and the travel control valve 9 kept in the neutral position, the hydraulic
braking force can be increased to brake the vehicle body appropriately and
an excessive temperature rise of the hydraulic fluid in the circuit can be
prevent. It is hence possible to prevent damages of hydraulic equipment
otherwise caused by an excessive temperature rise of the hydraulic fluid
in the circuit.
Further, since the timer function 32b is included in the controller 32 and
the throttle 33a as a hydraulic timer exists in the operative position of
the solenoid valve 33, a speed-down feeling at the high-speed gear just
after the travel control valve 9 is returned to the neutral position is
improved, and the cavitation otherwise caused upon switching of the travel
motor 7 to the large tilting can be prevented.
Additionally, since the capacity control of the travel motor 7 in
accordance with the switching of the solenoid valve 33 is performed only
when the transmission 13 is on the high-speed gear side, the worsening of
a speed-down feeling otherwise occurred when the transmission 13 is on the
low-speed gear side can be prevented.
A second embodiment of the present invention will be described with
reference to FIG. 6. In this embodiment, only the pilot pressure for
forward traveling is detected as the first detecting means for detecting
the operative state of the travel control valve 9. In FIG. 6, identical
members to those in FIG. 1 are denoted by the same numerals.
Referring to FIG. 6, in the pilot line 12a of the pilot operation circuit
12, a pressure switch 30a is only disposed which turns on when the pilot
pressure for forward traveling, that is produced as a pressure on the
secondary side of the traveling pilot valve device 11, exceeds a value
capable of shifting the travel control valve 9 from the neutral position
to the operative position. A signal from the pressure switch 30a is input
to the controller 32. A throttle 12c of a slow return valve 12e disposed
in the pilot line 12a has a smaller aperture than a throttle 12f disposed
in a pilot line 12b. For example, the aperture of the throttle 12f is the
same 1.4 mm as in the above first embodiment, whereas the aperture of the
throttle 12c is 0.8 mm.
In this embodiment, when the vehicle is traveling backward under the
ordinary running condition with the transmission 13 shifted to the
high-speed gear, the signal from the pressure switch 30a is off and the
signal from the low-speed gear select switch 14b is also off. Hence, the
solenoid valve 33 is shifted to the operative position as shown, and the
communication between the shuttle valve 35 and the control line 23c is
established. In this case, however, since the load pressure taken out by
the shuttle valve 23b is introduced to the control line 23c, the traveling
condition is not substantially different from that in the case of the
solenoid valve 33 being shifted to the closed position.
Further, since the throttle 12c in the pilot line 12a on the forward
traveling side has the smaller aperture, a difference in speed-down
feeling between forward and backward traveling can be reduced.
Accordingly, this embodiment can also provide advantages almost similar to
those in the first embodiment. In addition, since the pressure switch is
disposed only in the pilot line 12a on the forward traveling side, the
embodiment can achieve a reduction in the production cost of the traveling
control system.
It should be understood that while the minimum capacity of the travel motor
is set to a small value for increasing the speed of the hydraulically
traveling vehicle in the above embodiments, the traveling speed can also
be increased by setting the speed reducing ratio of the transmission to a
small value. The present invention is also applicable to such a modified
case with similar resultant advantages.
According to the present invention, since the capacity of the travel motor
is increased upon detection of that the travel control valve is in the
neutral position, the hydraulic braking force can be increased to brake
the vehicle body appropriately in downslope traveling, while increasing
the speed of the hydraulically traveling vehicle, and an excessive
temperature rise of the hydraulic fluid in the circuit can be prevented.
It is hence possible to prevent damages of hydraulic equipment otherwise
caused by an excessive temperature rise of the hydraulic fluid in the
circuit.
Also, since the capacity of the travel motor is increased when the travel
control valve is in the neutral position and the transmission is on the
high-speed gear side, the worsening of a speed-down feeling otherwise
occurred when the transmission is on the low-speed gear side can be
prevented.
Further, since the capacity of the travel motor is increased after the
elapse of a predetermined time after the detection of that the travel
control valve is in the neutral position, a speed-down feeling at the
high-speed gear just after the travel control valve is returned to the
neutral position is improved, and the cavitation otherwise caused upon
switching of the travel motor to the large tilting can be prevented.
Additionally, since the first detecting means comprises means for detecting
the operation signal for forward traveling only and the aperture of the
throttle for delaying return of the travel control valve from the forward
traveling position is set to be smaller than that of the throttle for
delaying return of the travel control valve from the backward traveling
position, it is possible to reduce a difference in speed-down feeling
between forward and backward traveling perceived when the travel control
valve is returned to the neutral position, and also achieve a reduction in
the production cost of the system.
It will be understood that changes in the details, materials, steps and
arrangements of parts which have been described and illustrated to explain
the nature of the invention will occur to and may be made by those skilled
in the art upon a reading of this disclosure within the principles and
scope of the invention. The foregoing description illustrates the
preferred embodiment of the invention; however, concepts, as based upon
the description, may be employed in other embodiments without departing
from the scope of the invention. Accordingly, the following claims are
intended to protect the invention broadly as well as in the specific form
shown.
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